Cystic fibrosis (CF) is a common lethal genetic disease that results from a mutation in the gene encoding a chloride channel protein, the CF transmembrane conductance regulator. As a result of this defect, epithelia within the body are impermeable to chloride ion transport (Boucher et al., Lung 161:1-17, 1983; Welsh, Physiol Rev 67:11443-1184, 1987). Although several organs are affected, including pancreas, intestine, and male genital tract, complications within the lung account for 95% of the morbidity and mortality (Means, M. Cystic Fibrosis: the first 50 years. In: Cystic Fibrosis—Current Topics Volume 1, edited by Dodge J A, Brock D J H, and Widdicombe J H. Chichester: Wiley and Sons, 1992, p. 217-250). In lung impaired by the disease, chloride transport into the airway lumen leads to excessive absorption of Na+ and fluid, reducing the volume of airway surface liquid (Jiang et al., Science 262:424-427, 1993). Desiccation of airway surface liquid leads to the concentration of mucin macromolecules, which are the gel forming constituents of mucus (Matsui et al., Cell 95:1005-1015, 1998). The viscoelastic properties of normal mucus are dependent on the concentration, molecular weight, and entanglements between mucin polymers (Verdugo et al., Biorheology 20:223-230, 1983). Further interaction of mucins with DNA (Potter et al., Am J Dis Child 100:493-495, 1960; Lethem et al., Am Rev Respir Dis 100:493-495, 1990; Lethem et al., Eur Respir J 3:19-23, 1990) and f-actin polymers (Sheils et al., Am J Path 148:919-927, 1996; Tomkiewicz et al., DNA and actin filament ultrastructure in cystic fibrosis sputum. In: Cilia, mucus, and mucociliary interactions, edited by Baum G L, Priel Z, Roth Y, Liron N, and Ostfeld E J. New York, N.Y.: Marcel Dekker, 1998) released from dying inflammatory cells is responsible for the dense and viscous nature of CF sputum. The inability to clear such mucus by cough or mucociliary clearance facilitates colonization of the lung with opportunistic pathogens.
While the etiology of CF lung disease can be attributed to the altered rheological properties of sputum, compromised lung function is rarely evident at birth. Instead, bronchiectasis and airway obstruction progress with age of patient. This chronic lung injury results from a persistent cycle of bacterial infection and inflammatory response. Airway damage results when neutrophils recruited into the lung release matrix degrading enzymes, such as elastase, and harmful reactive oxygen species (reviewed in Konstan and Berger, Pediatr Pulmonol 24:137-142, 1997).
Despite some promising advances, correction of CF by gene therapy is not yet attainable. Currently, antibiotic regimens coupled with drugs that facilitate the clearance of purulent airway secretions remain the mainstay treatments for progressive airway disease. Inhalation of purified rhDNase (Pulmozyme; Genentech, USA), which digests extracellular DNA present in the CF airway, is widely used as a respiratory decongestant. Such treatment is clinically effective for diminishing sputum viscosity and stabilizing the forced expiratory volume (FEV) (Fuchs et al., N Engl J Med 331:637-642, 1994). Other investigative therapies aimed at breaking down mucin or actin polymers, including N-acetylcysteine, nacystelyn (an N-acetyl-L-cysteine derivative), and gelsolin, can also reduce sputum viscosity experimentally, but have yet to attain clinical approval specifically for treatment of CF in the United States.
Therefore, there is a need in the art for improved therapeutic approaches for the treatment of cystic fibrosis, as well as other diseases and conditions that are associated with abnormally or excessively viscous or cohesive mucus or sputum.